1. Field of the Invention
The present invention relates generally to source follower circuits, and more particularly to integrated circuits (ICs) having source followers which comply with Low Voltage Differential Signaling (LVDS) standards.
2. Description of the Related Art
Physical layer interfaces are often a critical bottleneck in any application that requires high bandwidths, such as telecommunications and high-speed networking. Achieving high data transmission in a wireless base station, for example, is critical in.order to increase the variety of services offered with wireless telephonesxe2x80x94services such as e-mail, the Internet, and high-quality audio and video communications.
With its high speeds, low power, and low cost profile, Low Voltage Differential Signaling (LVDS) is one of the most promising technologies to address the physical layer interface. LVDS is a relatively new data interface standard that is defined in the Telecommunications Industry Association/Electronics Industry Association (TIA/EIA)-644 and the Institute of Electrical and Electronics Engineers (IEEE) 1596.3 standards. Essentially, it is a signaling method used for high-speed transmission of binary data over wirelines. The standard specifies a lower voltage swing than other transmission standards in order to provide higher data transmission speeds and inherently a greater bandwidth at a lower power consumption.
During operation, an LVDS driver converts a standard transistor-transistor logic/complementary metal-oxide semiconductor (TTL/CMOS) signal into a low-voltage differential signal. This differential signal is specified to travel at rates up to 655 megabits per second (Mbps) over media such as copper cables or printed circuit board (PCB) traces. An LVDS receiver receives and then translates this differential signal back into the TTL/CMOS signal. Since LVDS receivers respond only to differential voltages, they are relatively immune to noise such as common-mode signal reflections and electromagnetic interference (EMI) emissions are also reduced. LVDS also offers designers flexibility with the power supply voltage, as the technology is compatible at 5 volts, 3.3 volts, and lower. As a result, designers can reuse their LVDS solution even as systems move to lower voltages.
General purpose LVDS technology addresses point-to-point physical layer interfaces. These include intrasystem connections via printed circuit board traces or cables. The ultimate rate and distance of LVDS data transfer is dependent on the attenuation characteristics of the media and the noise coupling to the environment. Applications for general-purpose LVDS technologies include central office, PBXs, switches, repeaters, and base stationsxe2x80x94all in the telecommunications field. Such technologies are also used in hubs and routers in data communications, and other applications such as digital cameras, printers and copiers.
Outside the TIA/EIA-644 standard lies multipoint LVDS. Multipoint LVDS supports backplane applications, such as proprietary buses and small computer system interface (SCSI). SCSI is a high-performance peripheral interface that distributes data independently of the host computer and is commonly used with devices such as hard disk drives, tape drives, CD-ROMs, and scanners. In addition to general-purpose point-to-point and multipoint applications, LVDS has been used for several years as an interface to flat panel displays in notebook computers.
Although there are several advantages in adhering to the LVDS standard, the actual design of circuitry for low power and low noise is challenging. Conventional source follower circuits, for example, utilize both p-channel and n-channel transistor devices and provide a common mode voltage that is referenced to a positive supply voltage VDD. A resistor is typically used to set the output impedance. As a result of such conventional designs, large voltage swings on the chip exist which create high current spikes and thus high noise and power consumption. What is needed is a source follower circuit for LVDS which has a low power consumption, low noise, and the ability to drive a variety of different output loads. The source follower circuit should also have the ability to operate over variations in the power supply voltage.
According to the present invention, a source follower circuit for low voltage differential signaling (LVDS) has a low power consumption, low noise, and the ability to drive a highly capacitive load at an output port of an integrated circuit (IC). Advantageously, output signals of the circuitry are referenced to ground and are less affected by power supply variations. The source follower circuit includes a first p-channel transistor having a drain coupled to a first supply voltage and a gate coupled to a first input; a second p-channel transistor having a drain coupled to the first supply voltage and a gate coupled to a second input which is complementary to the first input; a third p-channel transistor having a gate coupled to the second input, a source coupled to a second supply voltage that is less than the first supply voltage, and a drain coupled to a source of the first p-channel transistor which forms a first output; and a fourth p-channel transistor having a source coupled to the second supply voltage and a drain coupled to a source of the second p-channel transistor which forms a second output which is complementary to the first output.